1. Technical Field
This invention is related to a process for defining vias through polyimide and silicon nitride layers for making circuit interconnections in an integrated circuit.
2. Background Art
In the fabrication of very large scale integrated circuits (VLSI), it is sometimes necessary to make circuit interconnections between layers of metallization which are separated by one or more layers of insulating material. In the case of an integrated circuit as shown in FIG. 1 in which contact metal layer 10 is sequentially covered by a layer 12 of silicon nitride and then a layer 14 of polyimide, it is necessary to define vias 16 through both of these layers to make circuit interconnections with additional metal layers that are subsequently deposited.
It is known in the art to define vias using the following process steps: (1) apply photoresist layer; (2) lithographically define the via pattern; and (3) etch the underlying layer with an appropriate wet or dry etchant. When necessary to define vias through two layers, the above process steps are repeated for the second layer.
The problem, however, is that with the tolerances inherent in current technologies, it is very difficult to align the vias in each of the respective layers. To ensure that the vias do coincide to the extent necessary for adequate metallization, it is necessary to make the vias in each layer significantly larger than what would ordinarily be necessary as seen in FIG. 1. In VLSI applications, this would consume a large amount of real estate on a semiconductor chip.
It is therefore desirable to have a process for defining vias through both a silicon nitride and a polyimide layer in a self-aligned manner such that the wasted space on the chip is avoided.